●Special Features 1: Coping with unprecedented natural disaster

The Necessity of Understanding the Levels ofWater Safety in Small and Medium Waterways
FUJITA Koh-ichi2
National Research Team into Measures for Evaluating the Maintenance of Small and Medium Waterways1

1. Foreword
Flood damage is becoming an increasing problem in all
regions in recent years, and the severe damage beingcaused by small and medium-sized waterways that areeasily affected by heavy downpours is especially noticeable. In addition to reawakening an awareness of the factthat water maintenance standards for small and mediumwaterways are still insufficient, this also indicates new aspects of fluctuations in natural conditions, such as increases in the frequency of heavy rain and changes in theway that rain is falling, and societal changes, such as alowering of our capability to cope with natural disasters
brought about by the ageing of society.
Small and medium-sized waterways tend to be relatively longer in comparison with the large rivers that are directly administered by the central government. For example, a first class river system that comes under theadministration of a prefecture may be anything up to alength of 77,000km. When considering this in relation to
the new aspects mentioned above from a standpoint ofprotecting the lives and property of local residents, there
is no question that placing as much emphasis on small andmedium-sized waterways is as important as it is for thelarge-scale rivers.
On the other hand, the river administrators and the autonomous bodies in charge of local disaster-prevent measures must accept responsibility for working together toestablish flood-control policies and crisis management
policies that match up with local situations and be effective from both the software and hardware point of views,
while at the same time dealing with the various related
situations, such as balancing tight financial restrictions
and ensuring the safety of not only main and subsidiarywaterways, but also the upper and lower reaches of therivers. However, the truth of the matter is that there is alack of fundamental information on flow capabilities andother data owing to large lengths of waterways being
overlooked when it comes to measurement surveys thatgauge longwise and crosswise channels, water levels,
flow rates and other elements.
Amidst this background, the National Institute for Landand Infrastructure Management (NILIM) urgently established the NILIM Research Team into Measures for Evaluating the Maintenance of Small and Medium Waterways
within the institute and set up a joint project with theRiver Bureau of the Ministry of Land, Infrastructure andTransport in order to develop a system for evaluating theflow capabilities of small and medium-sized waterways, as
is shown in Figure 1, and we also implemented the evaluation of flow capacities in first class rivers spread throughout the entire country and that come under the jurisdiction of prefectural governments in collaboration withvarious Regional Development Bureaus and other relatedbodies.
When it comes to evaluating the flows of small and medium-sized waterways, the shortage of fundamental data,
as mentioned above, erects a critical barrier. Owing tothis, it was decided that instead of the traditional data collection and analytic procedures, we should use an emergency passing grade strategy. In other words, we aimed atdeveloping a consistent method that would be inexpen-Figure 1 Small & Medium-Sized River Water Safety
Evaluation Flow

1. This article has been presented by the group leader under the organization of the team dedicated to its creation, but all of the following team members(including previous members) contributed to its authorship.
Supervisor: Minoru Kurishiro, Director of Waterways Research; Ryosuke Tsunaki, Director of the Crisis Management Technical Research Center
Former Supervisor: Jun Inomata, Former Director of Waterways Research; Nobuo Sugiura, Former Director of the Crisis Management Technical Research Center
Team Members: Takayuki Ishigami, Chief Research Officer of the Waterways Research Office;Tenri Nonaka, Chief Research Officer of the Water Pollution Research Office; Koichiro Umemura, Research Agency; Shu Otani, Researcher; Kenmei
Tsukamoto, Exchange Researcher
Former Team Members: Toshimutu Sasaki, Former Chief Research Officer of the Water Pollution Research Office; Koji Kawaguchi, Former Research
Officer of the Waterways Research Office
2. Manager of the Waterways and Environmental Research Office, Environment Research Division

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●Special Features 1: Coping with unprecedented natural disastersive, effective and simple as well as being available forpractical use within a few years at the most, and thatwould have the minimum accuracy level required by floodcontrol policy inspections for evaluating the flow capabilities of waterways that stretch to the lengths mentioned
above. Details on how this was achieved will be explainedlater.
The system that was developed, together with all related technical details, is scheduled to be made public for the
purpose of promoting the evaluation of flow capabilities inas large a number of small and medium-sized waterways
as possible, which will en-able the use of evaluation results once the technical meaning of passing grade has
been fully compre-hended, and which will enable evaluations to proceed with high scores if and when necessary.

and Transport in June 2005, and satisfies the minimum
conditions of 2m pitch for the laser measurement density,
which is a prerequisite for the measurement data.
(2) Importance of Filtering when Generating ThreeDimensional Land Form Data
In order to obtain three-dimensional land form data ofriver channels that can be used for discharge capacity
evaluations, it is necessary to remove all of the unnecessary elements that could cause problems during the discharge capacity calculations from the original data fromwhich noise has been eliminated, and create high-densityground data. This process is known as filtering, and thedifference between good selections and bad selections in
the filtering method for removing the unwanted elements
has a huge effect on accuracy.
First of all, the filtering process is run automatically on
the original data with a program that targets the entire
range of the measurement data in search of the major unwanted elements listed in Table 1, the data after automatic filtering is then compared against the Ortho image andthe unwanted elements removed (manual filtering) fromthe area of the waterways (ground items, such as structures and trees, etc.,) and the high-density ground data isthen created.

2. Eliminating Blank Areas in Waterway Land Form
Data Using Aerial Laser Measurements(1) Significance of Using Aerial Laser Measurements
It is a sad fact that we have had little success in surveying river channels in small and medium-sized waterways
because the areas that can be used for examining flood
control policies are restricted to fully-developed areas. Inorder to solve this problem and eliminate blank areas, wewill be using aerial lasers that can easily obtain high-density data on wide geographical areas.
As shown in Figure 2, aerial laser measurement is atechnology that acquires three-dimension land form databy shooting laser pulses into the air from an aerial laser
scanner mounted in an aircraft, and then analyzes the laser pulses that are bounced back. There are great hopesthat three-dimensional data will provide various types ofinformation, including the shape of cross-sectional river
channels and floodplains.
This evaluation system complies with the stipu-lations
laid out in the Policies for Three-Dimensional River
Channel Maps and Basins Using Aerial Laser Measurements proposed by the Ministry of Land, Infrastructure

Table 1 Major Filtering Items (proposed)
Transportation Road Facilities, Road Bridge (length 5m or longer,) Overhead Pass, PeFacilities
etc.
destrian Overpass
Railway Facilities Railway Bridge (length 5m or longer,) Overhead Pass(including overhead monorail lines,) Link-line Bridges
Foliage
Trees
Bamboo forests(3) System for Creating Cross-Sections of River
Channels
Using a Triangulated Irregular Network (TIN) model to

Vertical : Horizontal Ratio (1 : 3)

Figure 2 Conceptual Image of Aerial Laser Measurements

Figure 3 Creation of Cross-Sectional River Channel
Maps with TIN Data Evaluation Flow13●Special Features 1: Coping with unprecedented natural disastercreate cross-sectional maps of river channels concisely
and mechanically, we developed a system that automatically acquires the elevation over a random cross-sectional
measurement line that cuts across a line that marks thecenter of the river channel with the Triangulated Irregular Network created from the laser data, as can be seen inFigure 3(4) Considerations for Improving the Accuracy ofCross-Sectional River Channel Data
Apart from the discrepancies that occur with the aerial
laser measurements themselves (reliance is placed on themeasurement equipment and measurement implementation conditions, etc., but discrepancies of ±30cm for horizontal accuracy and ±15cm for vertical accuracy generally occurs) when acquiring the cross-sectional maps ofriver channels mentioned above, it is thought that the following four discrepancies also arise during the process ofcreating the cross-sectional maps.
・Discrepancies that occur through analyzing blanks inlaser data.
・Discrepancies that occur with interpolation from theTIN data.
・Discrepancies that occur with land element fil-tering.
・Discrepancies that occur because water surfaces cannot be measured.
In order to minimize these discrepancies, methods ofusing mostly winter measurements that are not so badly
affected by foliage growth, and carrying out accuracy
checks, correction and interpolation with the use of river
channel measurement cross-sections when they are available are being considered.

flow volume calculations acquired with precipitation intensity equations for all areas of the country createdthrough a unified method based on Amedasu rainfall dataand rational equations. When dams and other facilities forregulating flood water existed in the area, we applied themaximum amount of flow discharge to the dam discharge
data, and intend to reflect the results of this separately.
(2) Precipitation Intensity rThis uses the Amedasu Probable Precipitation Calculation Program developed by the Public Works ResearchInstitute, an independent administrative agency (http://www.pwri.go.jp/jpn/tech_inf/amedas/top.htm). This program creates the following pre-cipitation intensity equation (Fair equation) based on precipitation recorded between 1971 and 2003 in 748 of the approximately 1,300
Meteorological Agency s Amedasu measurement locations positioned throughout the entire country.
bT m
rtT �( t � a) n T
In this equation rt is the probable amount of rainfall(mm/hr) falling for t number of consecutive hours in theyear T, T is the probable year, t is the number of consecutive hours (hr) the rain will fall, and a, b, m, n, are the Fairparameters. The time a flood is likely to occur in the rational equation is applied to t.
(3) Flow Coefficient f and Flood Arrival Time t inthe Rational Equation.
The flow coefficient f used in the following rational
equation is weighted average of the flow coefficient foreach land use region (related to the area of each region,)and 0.7 is set for mountainous areas and 0.9 is set for flat
areas in accordance with the Waterway Erosion Prevention Technology Standards. The area of mountainous areas and flat areas are acquired through actual waterway
surveys.
1
Qp �
frA
3.6
In this equation Qp is the flow at the peak of flooding(m3/s), f is the flow coefficient, r is the precipitation intensity within the time that flooding occurs (mm/h), and A isthe area of the flow region (km2). The method of calculating the time of flood arrival can be made with either theKraven equation, the Sumiya equation, or the Doken
equation whichever is the most suitable.
(4) Calculation of One-Dimensional Non-uniform
Flow and Roughness Coefficient nAlthough the calculation methods for water levels thatcan be reflected back on the relative amount of information available on details on river channels, including riverbed materials and foliage within the area of the river, as
well as the complex and characteristic flood flows can beapplied practically, a large amount of time is required toaccumulate this data. Owing to this, in order to make thebest possible use of the afore-mentioned aerial laser measurements, the water level calculations from the waterway cross-sectional data explained earlier can be usedwithout modification if a rough coefficient calculated fromone-dimensional non-uniform flow calculations that use asingle composite rough coefficient in the cross-section isapplied to each section. However, as small and medium-3. Methods of Measuring Flow and Water Levels ofWaterways for which no Detailed Waterway
Plans are being Investigated(1) Basic Concept
We have an excellent opportunity to use the results
listed in waterway maintenance policies, waterway maintenance plans, overall plans and other documentation thatare based on the models required for actual water flow
and water level calculations by probable scale and the actual measurement data based on the hydrological parameters that these models use if they are available for thewaterways in question. However, there are many caseswith small and medium-sized waterways where this information is not available, so the problem of how to measurewater flow and water levels when no data on the resultsof detailed waterway investigations exist is a large one.
Owing to this and in response to the passing grade
strategy, we decided to apply simple calculations that that
do not rely on the fixed constant identifiers used with actual measurements. In consideration of the fact that detailed investigations will be carried out independently
when necessary in the future, we decided that it was necessary to create a foundation for calculating discharge capacities of small and medium-sized waterways at the very
least that could be used throughout the entire country on
a common basis as quickly as possible.
In more detail, this is a simple method of calculating
one-dimensional non-uniform flow with probable scale

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●Special Features 1: Coping with unprecedented natural disastersized waterways tend to have many steep slopes, it has
been designed so that calculations related to a combination of tranquil flows and rapid flows can also be used ifnecessary.
In cases like this, the method of applying the rough coefficients that include all necessary elements of the river
channels is extremely important. We consequently decided to select 58 rivers for which the rough coefficients are
estimated from actual measurements for even small andmedium-scale rivers, and analyzed the tendency of therough coefficients to discover the common denominator
for the component rough factor of small and mediumsized rivers, and calculated river levels based on this.

ment system into practical use for each regional bureau inorder to measure first-class river system throughout theentire country (109 river systems in total.) By providing
all national research agencies with the results of laser
measurements as they become available and by using thisflood safety evaluation system, we will be able to implement flood safety evaluation of small and medium rivers
within a minimal period of time.
Despite the tight fiscal conditions that have been prevalent in recent years, we hope that the results of this study
will contribute to enhanced levels of understanding offlood safety in small and medium-sized rivers from an accurate and objective point of view, leading up to a situation in which we can eliminate the risk of flood disasters
in areas with small and medium-sized rivers, and implement more effective methods of crisis management
through the use of various technologies and unique methods.

Altitude (T.P.m)

Altitude (T.P.m)

4. Expressing Flood Control Safety (Discharge Capacity,) etc.
By evaluating discharge capacities acquired from theresults of the water level calculations indicated in Figure 4,
we intend to create flood safety
evaluation maps for every river,Calculated Probable
Probable
Probable Percentage of Length that Satisﬁes Discharge Capacities
River Namecity, town, village and prefecture
LengthScale
Precipitation
FlowLeft Bank
Right Bank
similar to those shown in Figures
A Waterway, B River 8.6km
1/10 46.7mm/60mins 400m3/s
78%
85%
5 and 6.
We intend to gather together
Discharge Capacity
Discharge Capacity
the opinions of each local area
Satisfied
Not Satisfied
and of each regional bureau andriver control office in order to exLeft Bank
amine the thought-processes behind available budgets and local
Average River
Evaluated Flood
Water Level
BedPrevention Heightphilosophies with regard to thewaterways under their jurisdiction for the purpose of creating
Distance from Discharge (km)accurate flood-control evaluations, and this information will
Right Bank
then be reflected back onto theprocesses of determining evaluaEvaluated Flood
Average River
Water Level
Prevention HeightBed
tions for levees and displaying
the evaluations in an easy-to-unDistance from Discharge (km)derstand way.
5. Conclusion
Figure 4 Simulation of the Relationship between Water Level and EvaluatedWe are currently dedicated toFlood Prevention Height＊
putting the aerial later measure-

Figure 5 Expressing The Results of Water Safety Evaluations on Waterway Maps＊＊
The images in Figures4 to 6 are only for reference purposes, and are not

15Figure 6 Simulation of Maintenance Conditions for Local AutonomousBodies